Aqueous solutions of cobalt or cobalt and manganese also containing dissolved extraneous contaminant metals copper, iron, nickel and chromium can be obtained from the aqueous extraction of residues from the production of ortho, iso- or terephthalic acid by liquid phase oxidation of the corresponding xylene isomer with air in the presence of cobaltous ions or mixture of cobaltous and manganous ions. The residues extracted result from the recovery of the phthalic acid from the oxidation effluent and, if a solvent is used, the evaporation of the reaction solvent. The extraneous contaminant metals copper, iron, nickel and chromium have at different concentrations on cobalt adverse effects on the catalysis. For example, a copper concentration of 800 ppm of copper on cobalt substantially retards the oxidation of o-xylene while a 5000 ppm concentration of iron or nickel on cobalt has no adverse effect for o-xylene oxidation.
It is known that copper can be precipitated from such aqueous solutions (e.g., from U.S. Pat. No. 3,341,470) by treatment of the solution with hydrogen sulfide, a rather poisonous gas.
We have now discovered a less hazardous process for removing copper from such solution by a simple flow process. Such process is valuable for the removal of copper from aqueous solutions of cobalt wherever obtained.